The invention relates to novel liquid crystal compounds and more particularly to novel ferroelectric chiral smectic liquid crystals, which may be used as a display element material excellent in an image display response.
Liquid crystal display elements have been practically widely used in twisted nematic (TN) mode, guest-host (GH) mode and so on owing to the excellent properties thereof such as low voltage actuatability, low electric energy consumption, displayability in a thin structure and low eyestrain.
The liquid crystal display elements of nematic type, however, are disadvantageous in the slow speed of response in the order of several m sec - several tens of m sec, so that there are considerable limitations in the applied uses thereof.
In order to solve the problem referred to above, a satelite tracking network (STN) mode has been developed but it is still unsatisfactory in that it is necessary to precisely control of the cell gap and the tilt angle and in that the time of response is still low despite of that display properties such as display contrast and visual field angle have been considerably improved.
In order to comply with demands for novel liquid crystal displays superior in the time of response, ferroelectric liquid crystals have been provided for developing the liquid crystal devices having the far higher optical response time in the order of sec.
In the year of 1975, p-decyloxybenzylidene-p-amino-2-methylbutylcinamate (DOBAMBC) was synthesized as the ferroelectric liquid crystal by Meyer et al., and in the year of 1980 the high speed switching and memory properties of DOBAMBC were confirmed by Clark and Logawall [N. A. Clark et a., Appl. Phys. Lett. 36, 899 (1980)] so as to attract attention as an epoch-making liquid crystal element material capable of realizing the moving image display of a large picture in the simple matrix mode.
The molecular structure of the ferroelectric liquid crystals is characterized by comprising at least one assymetric carbon atom in the molecule thereof, having a large dipole moment and showing the smectic phase so that the apsis line of the molecule is tilted to the phase normal line by a some angle (tilt angle). When optically active compounds each having an optically active radical in the side chain fulfill these conditions, ferroelectricity and spontaneous polarization can be brought.
The spontaneous polarization and the speed of response of the ferroelectric liquid crystals are in the relation represented by ##EQU1## in which response speed, Ps; spontaneous polarization, E; impressed voltage and .eta.; rotation viscosity [M. A. Handschy, Appl. Phys. Lett, 41, 39 (1982)]. In order to realize the high speed of response, thus, it is necessary to develop compounds of a larger spontaneous polarization and a lower viscosity as far as possible.
As for the molecule design for causing a large spontaneous polarization, it has been proposed to suppress the free rotation of the liquid crystal molecule about the apsis line, arrange the assymetric carbon atom part and the dipole moment carrying group to be as close as possible with each other and introducing a group having a larger dipole moment directly in the asymetric carbon part.
Meanwhile, there are various problems to be dissolved for the practically usable ferroelectric liquid crystal display element e.g. in that it is difficult to attain unified orientation of the liquid crystal molecules, that a high degree of technique is necessary in order to control unified cell gaps in the order of 1-2 .mu.m and that the threshhold and hysteresis properties in the low-frequency range are not definite so that realization of the large picture and large capacity of display has been thought not to be so easy.